Back to EveryPatent.com
United States Patent |
5,581,079
|
Mickael
|
December 3, 1996
|
Epithermal neutron detector having enhanced efficiency
Abstract
A neutron detector sensitive principally to epithermal neutrons is
disclosed. In a preferred embodiment of the invention, a neutron counter
is substantially surrounded by a reflector composed of material having a
high neutron scattering cross section. The reflector is then wrapped in a
shield which is composed of a thermal neutron absorber. In a specific
embodiment of the invention, the counter, reflector and shield are mounted
inside a neutron moderator which can be composed of plastic.
Inventors:
|
Mickael; Medhat W. (Sugar Land, TX)
|
Assignee:
|
Western Atlas International, Inc. (Houston, TX)
|
Appl. No.:
|
394289 |
Filed:
|
February 24, 1995 |
Current U.S. Class: |
250/269.4; 250/390.1 |
Intern'l Class: |
G01V 005/10; G01T 003/00 |
Field of Search: |
250/390.01,390.1,269.4
|
References Cited
U.S. Patent Documents
4556793 | Dec., 1985 | Allen et al. | 250/267.
|
4581532 | Apr., 1986 | Givens et al. | 250/390.
|
4590370 | May., 1986 | Mills, Jr. et al. | 250/390.
|
Other References
Brochure: PDK-100: Enhance Formation Evaluation and Reservoir Monitoring
with New-Generation Pulsed Neutron Capture Technology: Copyright 1994
Western Atlas International, Inc.
|
Primary Examiner: Fields; Carolyn E.
Attorney, Agent or Firm: Fagin; Richard A.
Claims
What is claimed is:
1. A detector substantially sensitive to epithermal neutrons, comprising:
a neutron counter;
a neutron reflector substantially surrounding said counter, said reflector
having the characteristic of a high neutron scattering cross-section; and
a neutron shield substantially surrounding said reflector, said shield
primarily composed of material which substantially absorbs thermal
neutrons and allows passage of neutrons having energy above thermal level.
2. The detector as defined in claim 1 further comprising:
a neutron moderator substantially surrounding said reflector externally to
said shield, said neutron moderator comprising a material having a high
concentration of hydrogen nuclei in a chemical structure of said neutron
moderator material.
3. The detector as defined in claim 2 wherein said neutron moderator
comprises plastic.
4. The detector as defined in claim 1 wherein said neutron reflector
comprises carbon.
5. The detector as defined in claim 1 wherein said neutron reflector
comprises beryllium.
6. The detector as defined in claim 1 wherein said neutron reflector
comprises aluminum.
7. The detector as defined in any of claims 1, 4, 5, or 6 wherein said
neutron reflector has a thickness within a range of 0.5 to 0.9 inches.
8. A logging tool for making measurements of a characteristic of an earth
formation penetrated by a wellbore, comprising:
a source of high energy neutrons disposed within said tool;
a neutron detector substantially disposed within said tool at a
spaced-apart location from said source, said neutron detector sensitive to
epithermal neutrons, said neutron detector comprising a neutron counter,
said neutron counter generating an output responsive to the presence of
neutrons, said neutron detector comprising a neutron reflector
substantially surrounding said counter, said neutron detector comprising a
neutron shield surrounding said reflector, said neutron detector
comprising a neutron moderator substantially surrounding said shield
wherein said neutron counter, said neutron reflector and said neutron
moderator are respectively positioned so that substantially none of said
reflector and none of said moderator is interposed between said neutron
counter and said earth formation; and
means for recording said output of said counter.
9. The logging tool as defined in claim 8 wherein said neutron counter
comprises a helium-3 proportional counter.
10. The logging tool as defined claim 8 wherein said neutron reflector
comprises carbon.
11. The logging tool as defined in claim 8 wherein said neutron reflector
comprises beryllium.
12. The logging tool as defined in claim 8 wherein said neutron reflector
comprises aluminum.
13. The logging tool as defined in claim 8 wherein said neutron moderator
comprises a material having a high concentration of hydrogen nuclei within
a chemical structure of said neutron moderator material.
14. The logging tool as defined in claim 13 wherein said neutron moderator
comprises plastic.
15. The logging tool as defined in claim 8 wherein said source comprises a
pulsed neutron source capable of generating controllable bursts of high
energy neutrons.
16. A method of logging a wellbore penetrating the earth comprising the
steps of:
positioning a logging tool comprising a source of high energy neutrons
within said wellbore within a formation of interest;
irradiating said formation of interest with high energy neutrons from a
source disposed within said wellbore;
recording an output of a neutron detector disposed within said logging tool
at a spaced apart location from said source, said detector comprising a
neutron counter, a neutron reflector, a neutron shield, and a neutron
moderator, so that said output of said detector substantially corresponds
to a number of epithermal neutrons entering said detector from said
formation of interest.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to the field of radiation detectors. More
specifically, the present invention is related to a radiation detector
which is primarily sensitive to epithermal neutrons.
2. Discussion of the Relevant Art
Radiation detectors include devices capable of indicating the presence of
free neutrons. Devices known in the art for indicating the presence of
neutrons include helium-3 (He-3) proportional counters. He-3 counters
typically are filled with gas under pressure, the gas being primarily
composed of the isotope of helium having an atomic mass of 3 . Neutrons
entering the He-3 counter typically react with the gas in the counter in
such a way as to eventually cause the gas to ionize. When the gas in the
counter ionizes a measurable change is generated in an electrical voltage
applied to the gas through electrodes disposed in contact with the gas in
the counter. The output of the counter generally consists of voltage
pulses proportional in number to the number of neutrons detected by the
counter.
The He-3 proportional counter is primarily sensitive to thermal neutrons.
He-3 counters can also detect some epithermal neutrons, but at much lower
efficiency than the detection of thermal neutrons. The significance of the
types of neutrons and the relative efficiencies with which they are
counted by the He-3 counter will be further explained.
Epithermal and thermal neutrons can be generated by the interaction of
higher energy neutrons with atomic nuclei of other materials. For example,
materials such as earth formations which can be penetrated by wellbore
drilled into the earth, can be surveyed by a logging tool comprising a
source of high energy neutrons and one or more neutron detectors. Higher
energy neutrons can be generated by radioisotopic sources such as
Americium-b 241 surrounded by a beryllium shield, or by electrically
controllable pulse sources such as those described in, for example,
"PDK-100" (wellbore logging tool), Atlas Wireline Services, Houston, Tex.,
1992.
In such a wellbore logging tool, the high energy neutrons which are emitted
from the source typically collide with the atomic nuclei of the materials
forming the earth formations surrounding the wellbore. The neutrons will
typically lose some of their energy content with each collision until
their energy content reaches the epithermal level, and then the thermal
level, whereupon these neutrons can be detected by the He-3 counter.
Alternatively, thermal neutrons can be "captured" by atomic nuclei of
certain materials which may be present in the earth formation, such as
chlorine. Chlorine typically is present in earth formations in the form of
sodium chloride dissolved in connate water. Connate water can be contained
in voids, or pore spaces, which can form pan of some earth formations.
Certain properties of the earth formation can be determined by measuring
the numbers of neutrons detected by the He-3 counter at a plurality of
predetermined elapsed times from the generation of the high energy
neutrons by the pulsed source. Alternatively, a measurement can be made of
the number of neutrons detected by each one of a plurality of He-3
detectors positioned at spaced-apart locations from the source along the
tool. A plurality of detectors is typically used when the neutron source
is the Americium-241 type previously described, or is any other so-called
"steady-state" source.
A limitation of He-3 counters for determining properties of earth
formations is that He-3 counters, as previously stated, are primarily
sensitive to thermal neutrons. Also as previously stated, thermal neutrons
are subject to being "captured" by atomic nuclei of certain materials such
as chlorine which may be present in the earth formation. Capture of
thermal neutrons precludes their detection by the He-3 counter. The
numbers of thermal neutrons captured by thermal absorbers such as chlorine
in the earth formations is difficult to determine because the amount of
chlorine in the earth formation is typically not known at the time the
neutron tool is run in a wellbore. Unknown numbers of absorbed, and
therefore undetected, thermal neutrons can cause erroneous determinations
of the properties of the earth formations when the He-3 counter is used.
It is known in the art to determine the properties of the earth formation
by measuring the numbers of epithermal neutrons present in the earth
formation, the epithermal neutrons resulting from activation of the
formation with high energy neutrons. As previously explained herein,
epithermal neutrons have a higher energy content than thermal neutrons.
Epithermal neutrons are much less susceptible to capture by chlorine or
other materials which tend to capture thermal neutrons as a result of
their higher energy. Epithermal neutron measurements are therefore less
susceptible to error caused by undetermined chlorine concentration in some
earth formations.
Using epithermal neutrons to determine properties of the earth formation
requires a counter which is primarily sensitive to the epithermal
neutrons. An epithermal neutron counter is described, for example, in U.S.
Pat. No. 4,556,793 issued to Allen et al. The epithermal counter disclosed
in the Allen patent includes the He-3 counter previously described herein,
which is then enclosed first in a neutron moderating material, and then
further enclosed in a "shielding" material which is capable of
substantially preventing passage of thermal neutrons. The shielding
material stops most of the thermal neutrons which may be present in the
earth formation from entering the counter. Higher energy neutrons, which
can pass through the shielding material, are reduced in energy by the
moderating material. Reducing the energy of a neutron primarily means
slowing down or reducing velocity. The neutrons are slowed to an energy
level where they can be detected by the He-3 counter.
As previously explained, the He-3 counter is at least partially sensitive
to epithermal neutrons, and the thermal neutrons are excluded from the
counter by the shield, so the detector disclosed in the Allen patent
responds primarily to epithermal neutrons.
A limitation to the use of the epithermal neutron detector described in the
Allen patent is that the moderator changes the amount of time taken by the
neutrons to reach an energy level at which their detection in the He-3
counter can occur, relative to the amount of time taken by the neutrons to
reach the epithermal energy level only as a result of interaction with the
earth formation. Measurements which are related to the amount of time
taken for the high energy neutrons to slow down to epithermal energy
levels as a result of interaction with the formation, are therefore
distorted by using the detector disclosed in the Allen patent.
It is an object of the present invention to provide a neutron detector
which is primarily sensitive to epithermal neutrons which does not
significantly change the time distribution of the epithermal neutrons.
SUMMARY OF THE INVENTION
The present invention is a neutron detector sensitive principally to
epithermal neutrons. In an embodiment of the invention, a neutron counter
is substantially surrounded by a neutron reflector composed of material
having a high neutron scattering cross section. The reflector is then
wrapped in a shield which is composed of a thermal neutron absorber.
In a specific embodiment of the invention, the counter, reflector and
shield are mounted inside a neutron moderator which can be composed of
plastic.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a neutron tool disposed in a wellbore.
FIG. 2A shows an epithermal neutron detector according to the present
invention disposed in a wellbore logging tool.
FIG. 2B shows an end view of the assembly of the neutron detector according
to the present invention.
FIG. 3 is a graphic representation of Monte Carlo modeled detector counting
rate versus time for the detector according to the present invention for
different material compositions of a reflector with the detector
positioned in a zero porosity limestone earth formation.
FIG. 4 is a graphic representation of the detector counting rates as in
FIG. 3 with the detector positioned in a 20 percent porosity limestone
formation.
FIG. 5 is a graphic representation of the detector counting rates as in
FIG. 3 with the detector positioned in a 40 percent porosity limestone
formation.
FIG. 6 is a graphic representation of the detector counting rates with the
detector positioned in water, which is equivalent to 100 percent porosity.
FIG. 7 is a graphic representation of Monte Carlo modeled detector count
rates using reflectors of various thicknesses, with the detector
positioned in a zero porosity limestone formation.
FIG. 8 is a graphic representation of the detector counting rates as in
FIG. 7 with the detector positioned in 20 percent porosity limestone
formation.
FIG. 9 is a graphic representation of the detector count rates as in FIG. 7
with the detector positioned in 40 percent porosity limestone formation.
FIG. 10 is a graphic representation of the detector count rates as in FIG.
7 with the detector positioned in water.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The operation of the present invention can be better understood by
referring to FIG. 1. A logging tool 10 comprising a neutron detector 3 and
a source of high energy neutrons, shown at 22, is typically inserted into
a wellbore 8 penetrating an earth formation 14, the tool 10 being disposed
at one end of an armored electrical cable 1. The other end of the cable 1
is electrically connected to a recorder 2 which can store and process
signals generated by the detector 3 in response to detection of neutrons.
Neutrons 5 emanate from the source 22 and enter the wellbore 8. Some of the
neutrons 5 enter the earth formation 14 wherein the neutrons 5 collide
with atomic nuclei of the materials making up the earth formation 14. At
each collision, the neutrons 5 can change direction, and some of the
energy in the neutrons 5 is lost. Eventually, some neutrons 5 are reduced
in energy to a level where they can be detected upon entry into the
detector 3, the significance of which will be further explained.
The detector according to the present embodiment of the invention is shown
in FIG. 2A. The detector 3 according to the present invention can include
a neutron counter such as the previously herein described He-L3
proportional counter 12. It is contemplated that other neutron counters
known in the art such as lithium-6 glass counters can perform the same
function as the He-3 counter described herein. As previously explained,
the detector 3 is disposed within the logging tool 10 which is adapted to
traverse the wellbore 8 penetrating the earth formation 14.
The counter 12 is mounted within a housing 10A, which can contain signal
processing components (not shown) for transmitting measurements made by
the counter 12 to the recorder (shown as 2 in FIG. 1). The detector 3 is
mounted at an axially spaced apart location which is about eight inches
from the source 22, which is also disposed within the housing 10A. The
source 22 in the present embodiment is known in the art and is described
for example in "PDK-100", Atlas Wireline Services, Houston, Tex. 1993. The
source 22 periodically emits "bursts" of high energy neutrons. As
previously explained, the high energy neutrons interact with the wellbore
8, which may be filled with fluid (not shown), and also interact with the
formation 14 surrounding the wellbore 8.
In the present embodiment of the invention the counter 12 can be in the
shape of a cylinder 1.5 inches in diameter and six inches long. The
counter 12 can contain He-3 gas pressurized to ten atmospheres (about 145
PSIA).
The counter 12 is mounted within a reflector 16. The reflector 16 in the
present embodiment can be composed of a material such as beryllium,
carbon, aluminum or other material which has a high neutron "scattering"
cross-section. A high neutron scattering cross-section means that neutrons
interact with the atomic nuclei of the reflector 16 material by changing
direction, but losing only a small amount of their velocity, and thereby
their energy, with each interaction. Neutrons escaping from the counter 12
undetected tend to be returned to the counter 12 by interaction with the
reflector 16, whereupon the reflected neutrons have additional
opportunities to be detected by the counter 12.
The counter 12 is mounted eccentralized within the reflector 16 to enable
entry of neutrons into the counter 12 from the formation 14, and for the
counter 12 to be positioned as closely as possible to the earth formation
14. The positioning of the counter 12 within the reflector 16 will be
further explained. The counter 12 and reflector 16 are wrapped in a shield
20 which can be formed from sheet cadmium metal having a thickness of
about 0.025 inches. The shield 20 excludes passage of thermal neutrons
into the counter 12, but enables passage of epithermal and higher energy
neutrons. Since thermal neutrons are excluded from the counter 12, the
neutrons detected in the counter 12 are primarily epithermal neutrons,
even though the He-3 counter 12 detects epithermal neutrons at much lower
efficiency than it does thermal neutrons. By providing the reflector 14 to
return epithermal neutrons to the counter 12 which may escape the counter
12 undetected, the overall detection efficiency of the counter 12 to
epithermal neutrons can be increased.
The assembled counter 12, reflector 14 and shield 20 are mounted
eccentralized within a moderator 18. The moderator 18 can be composed of a
material, such as a plastic, for example polypropylene, which has a high
concentration of hydrogen nuclei in its molecular structure. The hydrogen
nuclei in the moderator 18 can be struck by high energy neutrons which may
enter from the wellbore 8. With each collision between a neutron and a
hydrogen nucleus, a large part of the momentum of the neutron is
transferred to the hydrogen nucleus. The moderator 18 therefore quickly
reduces the energy of neutrons which may enter the tool 10 from the side
of the tool 10 not impressed against the formation 14, so that these
neutrons will be captured by the shield 20 and not enter the counter 12.
The effect of the placement of the detector 3 within the moderator 18 is
to reduce the effect of the wellbore 8 on the measurement made by the
detector 3.
The relative mounting arrangement of the counter 12 within the reflector 16
and shield 20, and all of them then within the moderator 18, can be better
understood by referring to FIG. 2B. The counter 12 is positioned inside
the reflector 16 substantially axially parallel with the reflector 16. The
counter 12 is also eccentered within the reflector 16 so that the exterior
of the counter 12 intersects the exterior of the reflector 16
substantially on a line (not shown). The line is substantially parallel to
the axes of both the counter 12 and the reflector 16. The thickness of the
reflector 16, indicated at 16A, can be an amount between 0.1 inches and
0.9 inches for a counter 12 having 1.5 inch diameter. The significance of
the thickness 16A will be futher explained.
The counter 12 and the reflector 16 are themselves mounted eccentralized in
and substantially axially parallel with the tool housing 10A. The volume
external to the shield 20 but internal to the housing 10A is occupied by
the moderator 18, which as previously explained, can be composed of
plastic such as polypropylene, or other material having a high
concentration of hydrogen nuclei.
The counter 12, the reflector 16, the shield 20 and the moderator 18 are
assembled as described herein to form the detector 3. The relative
positioning as described herein of the counter 12, the reflector 16, the
shield 20 and the moderator 18 provide the detector 3 with the
characteristic of being directionally sensitive, which in the logging tool
10 of the present invention is primarily sensitive to epithermal neutrons
which enter from the formation 14.
SIMULATION TEST RESULTS
The present invention was tested by Monte Carlo simulation. Monte Carlo
simulation is known to those skilled in the art for modelling the response
of interactions of subatomic particles. The simulated configuration which
was tested is described in detail in the associated description of the
preferred embodiment and can be seen by referring to FIGS. 2A and 2B. In
all of the simulation test results described herein, the simulation
included a short duration "burst" of high energy neutrons, having an
equivalent energy of 14 million electron volts, emanating from the source
(shown as 22 in FIG. 2A) and interacting with the formation (shown as 14
in FIG. 2A).
Results of the tests are shown in FIGS. 3 to 10 as graphs representing
count rates from the detector 3 on the ordinate axes, and time elapsed
from the neutron burst on the coordinate axes.
FIG. 3 shows a comparison of count rates for no reflector around the
counter 12, shown at 24, and for carbon 26, beryllium 28, and aluminum 30
reflectors each having a 0.5 inch thickness (shown as 16A in FIG. 2B). The
count rates in the graph of FIG. 3 were determined for a limestone earth
formation (shown as 14 in FIG. 2A) having zero porosity.
FIG. 4 shows the results of a comparison of the effect of the various
reflector 16 materials on detector 3 count rates when the detector 3 is
positioned within a formation 14 having 20 percent porosity which is
filled with water. All of the simulated reflectors 16 have 0.5 inch
thickness. The curve representing no reflector is shown at 38. The
beryllium reflector curve is shown at 32, the carbon reflector response
curve at 34 and the aluminum reflector curve at 36.
FIG. 5 shows the results of a comparison similar to the comparisons shown
in FIG. 4 with the exception of the detector 3 being positioned within a
40 percent water-filled porosity limestone formation 14. The curve
representing no reflector is shown at 46. The aluminum reflector curve is
shown at 44, the carbon reflector curve at 42, and the curve for the
beryllium reflector at 40. FIG. 5 demonstrates that even within a high
porosity formation, the inclusion of the reflector 16 improves the count
rate of the detector 3.
FIG. 6 shows a similar comparison to that of FIG. 5 with the exception that
the detector 3 is positioned within water, or an equivalent 100 percent
porosity formation. The beryllium, carbon and aluminum curves
substantially match each other as shown at 48, 50, 52, respectively, and
all three curves representing count rates with a reflector are elevated
above the no reflector curve shown at 54. In water (100 percent porosity)
the improvement in count rate is reduced when compared with the increase
in count rate generated in a lower porosity formation 14, but the increase
in count rate is still visible on the graph in FIG. 6.
FIGS. 7 through 10 show the results of simulation of carbon reflectors
having varying thicknesses, the thickness values being zero (no
reflector), thin (0.1 inch), medium (0.5 inch) and thick (0.9 inch). The
simulations were conducted for limestone formations of zero, 20, 40 and
100 percent porosity as shown in the graphs in FIGS. 7, 8, 9 and 10,
respectively.
The results displayed in FIG. 7 indicate improved count detector rates as
the thickness of the reflector is increased from 0.1 inch shown at 62, to
0.5 inch shown at 58, to 0.9 inch shown at 56. The 0.1 inch reflector
exhibits reduced count rate relative to no reflector, shown at 60. This
result is consistent within all tested values of porosity, as shown
further in FIGS. 8 through 10. The explanation for this result is that a
greater number of the neutrons which would otherwise enter the counter
through the reflector are lost through interaction with the 0.1 inch
reflector than the number of neutrons which are returned to the counter by
internal reflection from within the counter. The counter with the 0.9 inch
reflector, as shown generally at 56, exhibits about double the count rate
of the counter with no reflector, shown at 60. For comparison, the result
with the 0.5 inch reflector is shown generally at 58.
FIG. 8 shows the results of a comparison similar to that shown in FIG. 7
with the exception of the detector being positioned within a limestone
formation having 20 percent porosity. The no reflector curve is shown at
68, and the 0.1, 0.5 and 0.9 inch reflector curves are shown at 70, 66,
and 64, respectively.
FIG. 9 shows a similar comparison to that shown in FIG. 8 with the
exception of the detector being positioned within a limestone formation
having 40 percent porosity. The no reflector response curve is shown at
76, and the 0.1 inch, 0.5 inch and 0.9 inch curves are shown at 76, 74,
and 72, respectively.
FIG. 10 shows a comparison similar to that shown in FIG. 9 with the
exception of the detector being positioned within water (equivalent to 100
percent porosity formation). The no reflector response curve is shown at
84, and the 0.1, 0.5, and 0.9 inch curves are shown at 86, 82, and 80
respectively.
In the tested cases having reflectors of 0.5 inch or greater thickness, the
addition of the reflector 16 increases the count rate of the detector 3
without substantially changing the time distribution of the neutrons.
While the present invention is directed to the use of the detector within a
wellbore logging tool, it is contemplated that other applications for
epithermal neutron detectors would be equally well served by construction
of a detector according to the present invention. The invention should be
limited in scope only by the claims appended hereto.
Top